Triarylphosphines having sulfonated aryl radicals are widely used as catalysts in the chemical industry, especially when dissolved in water together with rhodium compounds. They are successfully employed, inter alia. for hydroformylation of olefins (cf. DE 26 27 354 B1), cyclic amines are added onto conjugated dienes in their presence (cf. EP 176 398 A1), and they are used as catalysts for hydrogenation of organic compounds.
According to the teaching of DE 32 35 030 A1, sulfonated triarylphosphines are prepared starting from triarylphosphines, which are reacted with oleum at 0.degree. to 40.degree. C. When the reaction is complete, the reaction mixture is diluted with water, and the sulfonated triarylphosphine is extracted with a water-insoluble amine, dissolved in a water-insoluble organic solvent. The organic and the aqueous phases are separated from one another, and the sulfonated triarylphosphine is obtained from the organic phase, for example by treatment with an aqueous solution of an inorganic base. The aqueous phase essentially comprises dilute sulfuric acid (also called waste acid) having an acid content of 25% to 35% by weight. It is contaminated by water-soluble phosphorus compounds and other substances, which account for the COD (Chemical Oxygen Demand) value of about 5 to 25 g/l.
The COD value is a measure of the degree of contamination of waste waters. It is the amount of potassium dichromate, expressed as oxygen equivalents, consumed by the oxidizable contents of one liter of water. The COD value is determined by a standardized procedure which is described, for example, in Ullmanns Encyclopadie der Technischen Chemie (Ullman's Encyclopedia of Industrial Chemistry), 4th Edition (1981), Volume 6, pages 376 et sea. Water-soluble organic phosphorus compounds are to be understood as being sulfonated triarylphosphines, as well as secondary and degradation products thereof. If the sulfonated triarylphosphine is, for example, the sulfonation product of triphenylphosphine, the waste acid contains, inter alia, the phosphines
______________________________________ tri(m-sulfophenyl)phosphine (TPPTS) di(m-sulfophenyl)phenylphosphine (TPPDS) di(phenyl)(m-sulfophenyl)phosphine (TPPMS) the phosphine oxides formed by oxidation tri(m-sulfophenyl)phosphine oxide (TPPOTS) di(m-sulfophenyl)phenylphosphine oxide (TPPODS) ______________________________________
and, in small amounts, the phosphine sulfides formed by reduction.
______________________________________ tri(m-sulfophenyl)phosphine sulfide (TPPSTS) di(m-sulfophenyl)phenylphosphine sulfide (TPPSDS). ______________________________________
For economic reasons, endeavors are made to employ the waste acid in chemical processes directly rather than indirectly, i.e. by concentration, cleavage to give SO.sub.2, followed by catalytic oxidation of the sulfur dioxide. A precondition of this process is the elimination of the impurities dissolved in the waste acid.
It is known from EP 41 134 A2 that water-soluble salts of aromatic sulfonic acids can be separated from sulfonation mixtures with the aid of water-insoluble amines. For this purpose, the sulfonation mixture is diluted with water and then mixed with a solvent-free amine in an amount equivalent to the sulfonic acid. The sulfonic acid forms a lipophilic ammonium salt, which separates spontaneously from the aqueous phase. A precondition of this preparative process is that the sulfonation mixture is essentially free from unsulfonated starting material and other neutral substances. Waste products of chemical processes in general do not meet this requirement, since formation of small amounts of neutral substances by secondary and subsequent reactions cannot be prevented. Moreover, purification operations are not based on preparative processes, rather they are described as analytical processes in which even substances which are present in only trace amounts must be detected and separated out.